ELSA Cable User Manual
Page 61
Technical basics
ELSA MicroLink Cable
53
the actual information to be transmitted (useful data), the data packet also contains
address and control information.
IP address
IP addresses are used in IP networks for communications between various devices. In
this case, every host has its own unique address by which it can be identified
unambiguously. What does an IP address look like? It consists of four bytes separated
by dots, making a total of 32 bits. Each of the four bytes can take on values from 0 to
255, e.g. 192.168.130.124.
To be precise, the IP address refers to the interface, and not the host itself. A terminal
device with more than one interface (such as a router) has to have an IP address at its
disposal for every single interface. This is why ISDN routers from ELSA for example,
have an IP address for communication with the hosts in their own network, as well as a
second IP address for communication with the “outside world” using the ISDN. In the
same way, ELSA cable modems have an IP address for their own network and another IP
address for the exchange of data with the cable network.
Network
address
An IP address contains the address of the network as well as that of the host. The
network address is the same for all hosts on one network, whereas the address of the
host is exclusive and unique to the network. A router, for example, can have more than
one IP address, each one unique to the network.
Netmask
How then can you differentiate between the part that determines the network and the
part that identifies the host? With the netmask. You know what masks are: they cover
up one part of something and only allow a different part to be visible. This is exactly how
a netmask operates. It is a number which is identical in structure to the IP address, i.e.
32 zeros or ones. The netmask usually starts with ones at the beginning and ends with
zeros. The zeros at the end thus cover the part of the IP address which does not belong
to the network address.
Examples:
The same IP address, this time with another netmask:
You can see from this that an IP address alone is not enough. A host can only be
identified unambiguously in combination with a netmask.
This address...
...in bytes...
...looks like this in bits:
IP address
192.168.120.253
11000000.10101000.01111000.11111101
Netmask
255.255.255.0
11111111.11111111.11111111.00000000
Network address
192.168.120.0
11000000.10101000.01111000.00000000
This address...
...in bytes...
...looks like this in bits:
IP address
192.168.120.253
11000000.10101000.01111000.11111101
Netmask
255.255.0.0
11111111.11111111.00000000.00000000
Network address
192.168.0.0
11000000.10101000.00000000.00000000